Methacrylate moulding composition for obtaining decorative colored sheet material

ABSTRACT

A process for forming an acrylic sheet material comprises preparing a mixture comprising: (I) 20-89% wt of an acrylic polymer which is a copolymer containing 50-99% wt of methyl methacrylate unites and 1-50 wt % of a copolymerizable alkyl acrylate; (ii) 10-80 wt % of a mineral filler composition selected from the group comprising alumina trihydrate, magnesium hydroxide, talc and barium sulphate; and (iii) 1-40 wt % of decorative particles; and melt-moulding the mixture at a temperature above 150° C. to form a solid article. The process enables thermoformable acrylic articles which have the natural appearance of granite to be produced.

The present invention relates to moulded plastics materials containingfiller particles.

Polymeric materials may be formed in several different ways. Acrylicsheet materials are particularly useful for forming into articles suchas architectural cladding, bathtubs, shower enclosures, panelling etc.Filled curable moulding compositions based on acrylic polymers whichcomprise a polymerisable syrup of polymer in monomer, usually containinga high proportion of filler particles are widely used for moulding intokitchen sinks etc. The cured articles have superior properties impartedby the fillers in the moulding composition and the high molecular weightand crosslinking of the polymer. Although the casting process produces avery good product it is relatively expensive because each article mustbe made separately on a batch basis. Thick sheets of cured acryliccomposite material may also be made by a continuous casting process. Anexample of such a sheet is “Corian”™ sold by DuPont. These materials areuseful for making articles such as work surfaces, however they are rigidand not easily shapeable and usually must be cut and glued together toform 3-dimensional shapes. Also the continuous casting process requiresvery specialised processing equipment since the curing of cast syrupmust be done under controlled conditions to produce optimum productquality. There is therefore a need for a polymeric material which hassome of the benefits of filled acrylic composite materials but which maybe shaped by thermoforming techniques. There is also a need for a methodof making polymeric composite articles using cheaper processing methods.

According to the invention a process for forming an acrylic materialcomprises preparing a mixture comprising:

(i) 20-89 wt % of an acrylic polymer which is a copolymer containing50-99% wt of methyl methacrylate units and 1-50 wt % of acopolymerisable alkyl acrylate;

(ii) 10-80 wt % of a mineral filler composition selected from the groupcomprising alumina trihydrate, magnesium hydroxide, talc and bariumsulphate;

(iii) 1-40 wt % of decorative particles;

and melt-moulding the mixture at a temperature above 150° C. to form asolid article.

According to a second aspect of the invention we provide amelt-mouldable composition comprising:

(i) 20-89 wt % of an acrylic polymer which is a copolymer containing50-99% wt of methyl methacrylate units and 1-50 wt % of acopolymerisable alkyl acrylate or other methacrylate;

(ii) 10-80 wt % of a mineral filler composition selected from the groupcomprising alumina trihydrate, magnesium hydroxide, talc and bariumsulphate; and

(iii) 1-40 wt % of decorative particles.

The acrylic material may be further shaped by thermoforming techniquesinto useful articles such as contoured worktops or vanity unit surfaces,especially when the material is in the form of a sheet. Also multi-layerstructures may be made by laminating the acrylic composite sheet to asubstrate material or by coextruding the acrylic composite onto athermoplastic substrate or by extrusion coating onto e.g. a boardsubstrate.

The production of filled acrylic articles by melt processing is moreeconomical than casting and curing polymerisable compositions so thebenefits of a filled acrylic material may be achieved at relatively lowcost. The articles produced in this way have a solid look and feel andthe natural look of stone may be achieved by selecting appropriatecolours and particles.

The acrylic copolymer is a copolymer of methyl methacrylate. 50-99% byweight of the monomer units from which it is formed are derived frommethyl methacrylate monomer. The remaining units comprise one or morecopolymerisable alkyl acrylate or other methacrylate units, which mayinclude functionalised alkyl acrylates. Preferred acrylates includeC₁-C₈ alkyl acrylate, especially methyl, ethyl and butyl acrylate.Particularly preferred copolymers are derived from 70-95% methylmethacrylate and 5-30% of C₁-C₄ alkyl acrylate units. The resultingcopolymer preferably has a Tg of at least 60° C.

The acrylic copolymer may be made by any known polymerisation methods,especially solution, dispersion or bulk polymerisation. The polymer maycontain residues of a polymerisation initiator, or other processadditive. Preferred polymers also contain residues derived frompolymerisation controllers such as chain transfer agents which are addedto control molecular weight. Especially preferred additives includethose chain transfer agents which also enhance the thermal stability ofthe polymer such as mercaptans, e.g. alkyl mercaptans. Suitablemercaptans and polymerisation methods using mercaptans as chain transferagents in the production of acrylic polymers are already well known inthe art.

The acrylic copolymer is mouldable in its molten form, i.e. it is of agrade of acrylic polymer which is generally classed as suitable formoulding. The melt flow index (MFI) of the polymer is preferably in therange 0.5-25 g/10 mins at 230° C./3.8 kg (ASTM D-1238), more preferably0.8-16 g/10 mins. Typically the molecular weight is between 60,000 and180,000 preferably in the range 80,000 to 150,000 Mw, e.g.90,000-120,000.

The mouldable composition comprises 20-89, preferably 40-80% wt ofcopolymer.

The mineral filler is a particulate mineral compound selected fromalumina trihydrate (Al (OH)₃) (ATH), magnesium hydroxide (Mg(OH)₂), talcor barium sulphate. The particles preferably have an average particlesize of 5-100 μm, e.g. about 10-20 μm. The mouldable composition andresulting article may contain 10-80 wt % of the filler, preferably 10-50wt %. A mixture of more than one mineral compound may be used as thefiller.

The melt-mouldable mixture and resulting moulded article also containdecorative particles, e.g. coloured particles which provide a decorativeeffect to the sheet. Suitable particles include polymeric particles suchas coloured polyester or cross-linked acrylic particles of e.g. 5-5000μm average size. The decorative particles may contain colourants,opacifiers, fillers, plasticisers, stabilisers, lubricants etc. Themixture may contains 1-40% of such coloured particles, preferably 10-30%wt, more preferably 5-20% wt. A mixture of different coloured particlesmay be used to provide different appearances, e.g. to simulate theappearance of natural materials such as stone or granite.

The mixture may also contain other additives, such as stabilisers,toughening agents, lubricants, pigments and dyes. Suitable tougheningagents include rubbers e.g. MBS rubber, core-shell particle tougheningagents, e.g. graft copolymer core shell particles made from butylacrylate and MMA, SEBS, Kraton, or SBR etc. The toughening agent may bepresent in the polymer at levels of 1-50%, more typically 3-25% Apigment or dye may be present in the polymer prior to mixing the polymerwith the filler and other ingredients.

By melt-moulding we mean forming a shaped article by moulding underconditions, especially temperature, in which the polymer is molten andcan be caused to flow and then cooling below the melt temperature suchthat the mixture retains the shape formed when the polymer was molten.Preferred such methods include extrusion and injection moulding.Preferably the moulding temperature is less than 270° C. and is morepreferably 150-220° C. The moulding temperature must be maintained belowthe decomposition temperature of any of its constituents. For example,when ATH is used as the filler then the temperature should be maintainedbelow the decomposition temperature of ATH.

In one preferred form we provide a process for forming an acrylicarticle comprising the steps of feeding to an extruder a mixturecomprising:

(i) 20-89 wt % of an acrylic polymer which is a copolymer containing50-99% wt of methyl methacrylate units and 1-50 wt % of acopolymerisable alkyl acrylate;

(ii) 10-80 wt % of a mineral filler composition selected from the groupcomprising alumina trihydrate, magnesium hydroxide, talc and bariumsulphate;

(iii) 1-40 wt % of decorative particles;

and extruding the mixture at a temperature above 150° C. through a dieand subsequently cooling the extrudate to form a solid article.

In a second preferred form we provide a process for forming an acrylicarticle comprising the steps of forming a mixture comprising:

(i) 20-89 wt % of an acrylic polymer which is a copolymer containing50-99% wt of methyl methacrylate units and 1-50 wt % of acopolymerisable alkyl acrylate;

(ii) 10-80 wt % of a mineral filler composition selected from the groupcomprising alumina trihydrate, magnesium hydroxide, talc and bariumsulphate;

(iii) 1-40 wt % of decorative particles;

and injection-moulding the mixture at a temperature above 150 ° C into amould to form a solid article.

A preferred form of article is a sheet, which may vary in thicknessbetween <1 mm and about 20 mm, e.g. 1-10 mm. Many other shapes ofarticles are possible, especially when using injection mouldingtechniques to melt mould the article.

We also provide a thermoformable article, preferably in the form of asheet, comprising

(i) 20-89 wt % of an acrylic polymer which is a copolymer containing50-99% wt of methyl methacrylate units and 1-50 wt % of acopolymerisable alkyl acrylate;

(ii) 10-80 wt % of a mineral filler composition selected from the groupcomprising alumina trihydrate, magnesium hydroxide, talc and bariumsulphate;

(iii) 1-40 wt % of decorative particles.

The invention will now be further described in the following examples.

EXAMPLE 1

1600 g of a copolymer of methyl methacrylate (PMMA) with about 12 wt %ethyl acrylate having an average molecular weight (Mw) of around 90,000(by GPC using PMMA standards) and MFI of 13 g/10 min as measured at 230°C./3.8 kg was mixed with 400 g (20 wt %) of fine mineral particles ofalumina trihydrate (ATH) with mean particle size of 8 μm, and 300 g (13wt % ) of a 1:1 mixture of black and white decorative coloured polyesterparticles having an average particle size of 0.1 to 0.6 mm (supplied bythe J. Marshall company). The mixture was dry blended at first and thendelivered to an extruder and passed through a die at 180° C. to shape itinto a 3 mm thick flat sheet. It was cooled and slightly pressed betweentwo rotating rollers which were operating at room temperature. Theresulting extruded tape had the appearance of a granite material.

EXAMPLE 2

Example 1 was repeated with 1 wt % of lubricant PTFE particles added tothe mixture at the dry blending operation stage. The new mixture wasextruded into 3 mm sheet using an average extrusion temperature of 170°C.

EXAMPLE 3

5 w % of MBS toughening agent was added to the mixture in Example 1, dryblended and extruded at 180° C. A sheet with natural granite appearancewas made. Its properties are given in Table 1.

EXAMPLE 4

The base polymer used in Example 1 was pigmented by the use of knownmineral based pigments frequently used in conjunction with acrylicsproducts. Samples having the appearance of various granites from whiteto pink, to grey, to red were made.

EXAMPLE 5

The ATH filler used in Example 1 was replaced with a more thermallystable magnesium hydroxide material (at 20 wt %) with an averageparticle size of about 2-5 microns. A 3 mm thick sheet with graniteappearance was made.

EXAMPLE 6

Example 1 was repeated but coloured polyester decorative particles withmuch larger particle size in the range 1 to 1.4 mm were used.

EXAMPLE 7

Preparation of Thermoformed Article

The sheet made in Example 1 was thermoformed into a small cup using thefollowing conditions:

Composite polymer sheet of 3 mm thickness was cut to 14 cm×16.5 cm toallow it to be secured onto the mould. The mould and polymer assemblywere then placed in an electric oven preheated at 170° C. After 30minutes a two stage rotary vacuum pump was attached to the mould andmaximum vacuum applied. Once thermoforming had been completed the ovenheating was turned off and vacuum continued to be applied until the PMMAsurface temperature had dropped to 80° C. A cup with diameter of 60 mmand depth of 25 mm was successfully vacuum formed from this material.

EXAMPLE 8

Example 1 was repeated but instead of extruding the mixture it wasinjection moulded into test pieces, using a conventional injectionmoulding machine with the barrel temperature setting of around 190° C.The test pieces had the appearance of granite, although the decorativeparticles were not uniformly distributed. Some of their properties arelisted in Table 1.

EXAMPLE 9

Example 8 was repeated using the toughened recipe of Example 3. A testpiece having a granite appearance was successfully produced.

EXAMPLE 10

A mixture comprising 40% ATH, 48% of the acrylic polymer used in Example1 and 12% of the decorative particles (smaller size) were dry blendedand successfully injection moulded at 190° C.

EXAMPLE 11

A mixture comprising 30% magnesium hydroxide (average particle sizeabout 2-5 microns), 58% of the acrylic polymer used in Example 1 and 12%of the decorative particles (smaller size) were dry blended andsuccessfully injection moulded at 190° C. The Charpy impact strength wasmeasured as 10.52 kJm⁻².

EXAMPLE 12

A mixture comprising 10% magnesium hydroxide, 78% of the acrylic polymerused in Example 1 and 12% of the decorative particles (smaller size)were dry blended and successfully injection moulded at 190° C. TheCharpy impact strength was measured as 17.94 kJm⁻².

Table 1 shows the measured properties of the samples made in some of theExamples. Flexure testing was in accordance with ISO 178 and impacttesting was in accordance with ISO 179. The properties of a compositeproduct containing about 60 wt % ATH in PMMA made by the cell castprocess are also shown for comparison.

TABLE 1 Flex Mod Flex Strength Displacement At Sample (Gpa) (Mpa) Break(mm) Control (cast) 8.88 67.32 1.03 Example 1 3.54 69.20 2.82 Example 32.08 35.00 2.49 Example 8 4.14 64.92 1.80 Example 9 3.66 61.58 2.60

What is claimed is:
 1. A process which comprises: preparing a mixturecomprising (i) 20 to 89 wt %, relative to the total weight of saidmixture, of an acrylic material consisting essentially of an acrylicpolymer which is a copolymer containing 50 to 99 wt % of methylmethacrylate units and 1 to 50 wt % of alkyl acrylate units; (ii) 10 to80 wt %, relative to the total weight of said mixture, of a mineralfiller selected from the group consisting of alumina trihydrate,magnesium hydroxide, talc and barium sulphate; and (iii) 1 to 40 wt %,relative to the total weight of said mixture, of decorative particles;and melt-molding the mixture at a temperature above 150° C. to form asolid acrylic article.
 2. A process as claimed in claim 1, wherein saidcopolymer comprises 70 to 95 wt % methyl methacrylate units and 5 to 30wt % of C₁-C₄ alkyl acrylate units.
 3. A process as claimed in claim 1,wherein said copolymer has a Tg of at least 60° C.
 4. A process asclaimed in claim 1, wherein the mixture contains (iii) 5 to 25 wt % ofdecorative colored particles.
 5. A process as claimed in claim 1,wherein the molding temperature is in the range of above 150 to 220° C.6. A process as claimed in claim 1, wherein said melt molding is carriedout by extruding the mixture through a die.
 7. A process as claimed inclaim 1, wherein said melt molding is carried out by injection moldingsaid mixture into a mold.
 8. A process as claimed in claim 1, whereinsaid mixture comprises (i) 40 to 80 wt %, relative to the total weightof said composition, of said acrylic polymer; (ii) 10 to 50 wt %,relative to the total weight of said composition, of said filler; and(iii) 10 to 30 wt %, relative to the total weight of said composition,of said decorative particles.
 9. A process as claimed in claim 1,wherein said mixture comprises (i) 40 to 80 wt %, relative to the totalweight of said composition, of said acrylic polymer; (ii) 10 to 50 wt %,relative to the total weight of said composition, of said filler; and(iii) 5 to 20 wt %, relative to the total weight of said composition, ofsaid decorative particles.
 10. A moldable composition comprising: (i) 20to 89 wt %, relative to the total weight of said composition, of anacrylic material consisting essentially of an acrylic polymer which is acopolymer containing 50 to 99 wt % of methyl methacrylate units and 1 to50 wt % of alkyl acrylate units; (ii) 10 to 80 wt %, relative to thetotal weight of said composition, of a mineral filler selected from thegroup consisting of alumina trihydrate, magnesium hydroxide, talc andbarium sulphate; and (iii) 1 to 40 wt %, relative to the total weight ofsaid composition, of decorative particles; wherein the molecular weightof the acrylic polymer is in the range of 60,000 to 180,000.
 11. Amoldable composition as claimed in claim 10, wherein said copolymercomprises 70 to 95 wt % methyl methacrylate units and 5 to 30 wt % ofC₁-C₄ alkyl acrylate units.
 12. A moldable composition as claimed inclaim 10, wherein said copolymer has a Tg of at least 60° C.
 13. Amoldable composition as claimed in claim 10, which contains (iii) 5 to25 wt % of decorative colored particles.
 14. A moldable composition asclaimed in claim 10, further comprising 1 to 30% by weight, relative tothe total weight of said composition, of a toughening compound.
 15. Amoldable composition as claimed in claim 10, comprising (i) 40 to 80 wt%, relative to the total weight of said composition, of said acrylicpolymer; (ii) 10 to 50 wt %, relative to the total weight of saidcomposition, of said filler; and (iii) 10 to 30 wt %, relative to thetotal weight of said composition, of said decorative particles.
 16. Amoldable composition as claimed in claim 10, comprising (i) 40 to 80 wt%, relative to the total weight of said composition, of said acrylicpolymer; (ii) 10 to 50 wt %, relative to the total weight of saidcomposition, of said filler; and (iii) 5 to 20 wt %, relative to thetotal weight of said composition, of said decorative particles.
 17. Amolded article in the form of a sheet of material formed by molding amoldable composition as claimed in claim 10 in its molten state andsubsequently cooling, said article being further shapeable above itssoftening point.
 18. A molded article as claimed in claim 17, whereinthe sheet of material is formed by molding the moldable composition at atemperature above 150° C.
 19. A shaped article obtained by thermoforminga molded article as claimed in claim
 17. 20. The moldable composition ofclaim 10, wherein said alkyl acrylate units are selected from the groupconsisting of methyl acrylate, ethyl acrylate, and butyl acrylate.
 21. Aprocess as claimed in claim 1, wherein the solid acrylic article is inthe form of a sheet.
 22. A process as claimed in claim 1, wherein themolecular weight of the acrylic polymer is in the range of 60,000 to180,000.
 23. A process as claimed in claim 1, wherein the mineral filleris alumina trihydrate.
 24. A process as claimed in claim 1, wherein themineral filler is magnesium hydroxide.
 25. A process as claimed in claim1, wherein the mineral filler is talc.
 26. A process as claimed in claim1, wherein the mineral filler is barium sulphate.
 27. A moldablecomposition as claimed in claim 10, wherein the mineral filler isalumina trihydrate.
 28. A moldable composition as claimed in claim 10,wherein the mineral filler is magnesium hydroxide.
 29. A moldablecomposition as claimed in claim 10, wherein the mineral filler is talc.30. A moldable composition as claimed in claim 10, wherein the mineralfiller is barium sulphate.